Direct imaging monochromatic electron microscope

ABSTRACT

A direct-imaging, monochromatic electron microscope includes an objective lens for collecting a substantial portion of emitted electrons from an area across a sample surface, a first transfer lens for collimating the electrons into beams, an energy filter receptive of the beams to transit monochromatic beams, and a second transfer lens receptive of the monochromatic beams for refocusing the same through a projection lens to effect an image of the plurality of spots in a projection plane. The objective lens is formed of a magnetic toroidal coil having a central hole therein with a dish-shaped magnetically permeable member cupped coaxially over the toroidal coil. The permeable member has a neck portion protruding through the central hole. The sample surface is interposed proximate the objective lens between the objective lens and the energy filter.

The present invention relates generally to the field of electronmicroscopes and particularly to a direct imaging monochromatic electronmicroscope useful for X-ray photoelectrons and Auger electrons.

BACKGROUND OF THE INVENTION

A variety of electron microscopes and associated surface analyzers haveevolved in recent years. General background is given, for example, inIntroduction to Analytical Electron Microscopy, Plenum Press (New York1979). A popular type is a scanning electron microscope in which afocused electron beam is scanned over a sample surface with secondaryelectrons being detected in correlation with scanning position andprocessed electronically to provide a picture of topographical features.Associated mapping of chemical constituents in the surface is achievedwith characteristic X-rays produced by the electron beam. However,resolution from the X-rays is not commensurate with the topographicalresolution. Also, X-rays are not suitable for detecting elements withlow atomic numbers or for near surface sensitivity.

Another method for analyzing surfaces is with secondary Auger electronsgenerated at the sample surface by the focused primary electron beam.Auger microprobes are suitable for detecting elements with low atomicnumbers and have sensitivity to a few atomic layers. Surface mapping ofelements is accomplished by scanning with the primary electron beam. Anexample of a scanning Auger microprobe is provided in U.S. Pat. No.4,048,498. Scanning Auger is limited in analysis area to about 500angstroms diameter by scattering of the primary beam in the surfaceregion.

Another approach to surface analysis is electron spectroscopy forchemical analysis (ESCA) which involves irradiating a sample surfacewith X-rays and detecting the characteristic photoelectrons emitted. Thephotoelectrons are filtered by electrostatic or magnetic means whichallow only electrons of a specified energy to pass through. Theintensity of the resulting beam reflects the concentration of a givenchemical constituent of the sample surface. U.S. Pat. Nos. 3,617,741 and3,766,381 describe such a system. Chemical mapping of the surfacerequires moving a component or aperture to detect electrons from variousparts of the surface, since X-rays generally cannot be focusedsufficiently onto small areas of the surface to allow scanning with highresolution.

Therefore, continuing efforts have been directed toward direct imagingof characteristic emissions. One approach is described in "PhotoelectronMicroscopy--Applications to Biological Surfaces" by O. Hayes Griffith,presented at a symposium "Small Area Solid and Surface Analysis" in NewOrleans, Feb. 25-Mar. 1, 1985. The system described therein images lowenergy photoelectrons from ultra-violet radiation. It is acknowledgedtherein that an elemental analysis is not provided. Also, imaging withhigher energy electrons has been less successful because of aberrationsthat become more prominent.

The Griffith document, on Page 16, describes a further approach in whichemitted electrons are focused with spiral trajectories along magneticflux lines. Resolution depends on the diameter of the spiral, and it ispointed out that the main limitation is that maximum magnification, andtherefore resolution, is low.

Energy filtering of the electron beams is important to obtain amonochromatic beam characteristic of an element being analyzed.Electrostatic filtering may be achieved with concentric hemisphericalconductors having an applied voltage therebetween, such as described inaforementioned U.S. Pat. No. 3,766,381.

A magnetic system for filtering electrons is described in "Modificationof a Transmission Electron Microscope to Give Energy-Filtered Images andDiffraction Patterns, and Electron Energy Loss Spectra" by R. F.Egerton, J. G. Philip, P. S. Turner and M. J. Whelan, Journal of PhysicsE: Scientific Instruments, Volume 8, 1033-1037 (1975). This referencedescribes a transmission electron microscope. The energy filter is amagnetic prism cooperative with an electrostatic mirror to transit theelectrons twice through the prism. Direct imaging of surface elements isachieved with relatively high energy electrons (e.g. 80 kev). However,although the filtered energies of transmitted electrons arerepresentative of elemental constituents, this instrument utilizesspecially prepared thin film samples in transmission and is not intendedto analyze solid surfaces.

An instrument for imaging secondary ions from surfaces is described in"Secondary Ions Microanalysis and Energy-Selecting Electron Microscopy"by R. Castaing, Electron Microscopy in Material Sciences, Academic Press(New York 1971) pages 1033/8161. This instrument accelerates very lowenergy (e.g. 10 ev) secondary ions from surfaces and performs mass andenergy analysis while simultaneously forming two dimensional images.

A variety of electrostatic and magnetic electron lenses are known asdescribed, for example, in the aforementioned text by Hren et al. Onesuch magnetic lens is a single pole piece lens described in Hren et al.on Pages 68-69 as pancake and snorkel lenses. Further details of snorkellenses are given in "Some Properties of Single Pole Piece ObjectiveElectron Lenses" by S. M. Juma, M. A. A. Khaliq and F. H. Antar, Journalof Physics E: Scientific Instruments, Volume 16, 1063-1068 (1983).Single pole piece lenses apparently have not evolved to be useful inpractical electron microscopy.

In view of the foregoing, a primary object of the present invention isto provide an electron microscope for two dimensional imaging ofmoderate energy (50 to 3000 ev) secondary electrons emitted from solidsurfaces, where the emitted electrons are monochromatized in theprocess.

Another object is to provide a novel direct imaging microscope that isparticularly useful for X-ray photoelectron chemical analysis ofsurfaces.

A further object is to provide a novel direct imaging microscope that isparticularly useful for chemical mapping with Auger electrons.

Yet another object is to provide a monochromatic electron microscopehaving improved collection efficiency of electrons from a samplesurface.

SUMMARY OF THE INVENTION

The foregoing and other objectives are achieved according to the presentinvention by a direct-imaging, monochromatic electron microscope, forexample for Auger electrons or x-ray photoelectrons. The microscopeincludes emitting means for emitting electrons from at least one andpreferably a plurality of spots across a sample surface, objective meansfor collecting a substantial portion of the emitted electrons from thesample surface and collimating means for collimating the substantialportion of the emitted electrons into a plurality of groups of electronbeams. The microscope further includes an energy filter receptive of thegroups of beams to transit monochromatic beams having a selected energy,imaging means receptive of the monochromatic beams for focusing the sameto effect an image of the plurality of spots, and detector means fordetecting the image.

The objective means comprises a magnetic objective lens and preferably asingle pole piece objective lens formed of a magnetic field generatingtoroidal coil with a dish-shaped magnetically permeable member cuppedcoaxially over the toroidal coil. The permeable member more preferablyhas a neck portion extending through a central hole in the coil. Theobjective lens is situated to collect the substantial portion of theemitted electrons from a sample surface that is interposed proximate theobjective lens and is most preferably between the objective lens and theenergy filter.

Suitable for the energy filter is a spherical analyzer of hemisphericalconfiguration with an entrance aperture receptive of the electron beamsfrom the focusing means and a slotted exit aperture locateddiametrically opposite the entrance aperture. The electron microscopealso should include means for selecting the energy for the monochromaticelectron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electron microscope havingcomponents according to the present invention.

FIG. 2 is an axial cross section of an objective lens utilized in theelectron microscope of FIG. 1.

FIG. 3 is an axial cross section of a transfer lens utilized in theelectron microscope of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

A direct imaging monochromatic electron microscope according to thepresent invention is illustrated schematically in FIG. 1. The systemcomponents are in appropriate enclosures (not shown) so as to operate athigh vacuum. The preferred purpose of the microscope is to display a twodimensional map of a selected elemental constituent according to itsconcentration at or very close to a surface 12 of a sample 14. Thesurface is subjected in the conventional manner to a beam 15 from anenergy source 16, preferably a source of electrons or X-rays.

In the case of an electron beam source it is especially useful that thebeam be of suitable energy to cause Auger electrons to be emitted fromthe sample surface. With incident X-rays, photoelectron emission willoccur and be utilized. With either of these sources or other similarsituations of electron emission or transmission, the electron microscopeof the present invention filters the energies of the electrons toproduce and image a packet of monochromatic beams having a selectedenergy characteristic of a constituent element being mapped.

The emitted electrons from surface 12 are collected by a magnetic typeof objective lens 20 which is preferably of the single pole piece type,such as a pancake lens and most preferably a snorkel lens such asdisclosed in the aforementioned pages 68-69 of Hren et al. and thearticle by Juma et al. (As used herein and in the claims, "lens" refersto an electron optical component.) Such a snorkel lens is illustrated inFIG. 2 of the present application. The snorkel objective lens includes amagnetic field generating toroidal coil 22 of wire having a central hole24 therein. Wire leads 26 from the coil are connected to a currentsource 28. For, example, the coil has 500 turns, and a current of 1ampere is used.

A dish-shaped magnetically permeable member 30 is cupped coaxially overtoroidal coil 22. The permeable member has a neck portion 32 with a tip36 extending substantially through central hole 24 of the coil. Asexample the outer diameter of the neck is 20 mm and its overall lengthis 30 mm. Coolant may be needed, in which case cover plate 38 andsemitoroidel member 41 of non-magnetic material such as aluminum withO-ring seals 39 cooperates with permeable member 30 to enclose coil 22in a coolant chamber 40 with an inlet pipe 42 and an outlet pipe 44 forthe coolant to disperse heat generated in the coil by the current.

Tip 36 of the neck of the lens is positioned toward sample surface 12.The sample, in this preferred embodiment, is outerposed between neck tip36 and focus plane 54 as well as subsequent components to be described.Because of the characteristics of this lens, a very wide solid angle ofreception of electrons from surface 12 is practical, even approaching60°. The magnetic field of the single pole lens is such that theelectrons are focused in a first image plane 54 (FIG. 1). Amagnification of about 10 is preferred.

According to principles of electron optics, groups of electron beams arefocused by objective lens 20 through an objective aperture 55 into imageplane 54 in a pattern in correlation with various electron-emittingspots on the sample surface. Two such groups 56,60 are shown in FIG. 1,as solid and broken lines respectively. These are shown tracing throughmost of the system (the broken lines are partially omitted for clarity).Aperture 55 limits aberrations from the objective lens. An orifice 61 inimage plane 54 selects the electron beams for the area on surface 12 tobe mapped. Although not shown, the objective lens system may includeadditional magnetic or electrostatic lens components of the desired orconventional type between the single pole piece lens 20 and orifice 61,particularly for Auger imaging where more magnification is often needed.

A collimating transfer lens 62 is spaced beyond image plane 54 adistance corresponding to the focal distance of the transfer lens so asto refract each group into a group of parallel electron beams. Thetransfer lens may be any conventional, high quality type lens, eitherelectrostatic or magnetic, but should have a total coefficient ofaberration less than 50 cm, and preferably less than 30 cm.

A particularly desirable transfer lens 62 is depicted in FIG. 3 and isof the type described in a document entitled "An AsymmetricElectrostatic Lens for Field Emission Microprobe Applications" by J.Orloff and L. W. Swanson (June 1978). A washer-shaped first component 64with a first orifice 66 therein constitutes the inlet side for thebeams. A cup-shaped second component 68 has a flat cup-bottom portion 70spaced coaxially proximate to first component 64. A washer-shaped thirdcomponent 2, similar to component 64, is spaced coaxially proximate therim 74 of cup-shaped second component 68. The first, second and thirdcomponents have coaxial orifices 66,76,78 of similar diameters forcooperatively passing the electrons therethrough. Lens 62 desirably isoperated with the first and third components 64,72 at earth potential(i.e., generally the potential of enclosures for the system, not shown),and second component 68 at a negative voltage relative to the earthpotential, from a voltage source 80.

Referring again to FIG. 1, an energy filter 82 is receptive of theelectron beams to pass through (transit) corresponding monochromaticbeams having a selected energy. The filter is of the known r desiredtype such as a prism-mirror system shown in the aforementioned articleby Egerton et al. However, particularly for x-ray photoelectrons, thefilter preferably is a spherical analyzer of conventional hemisphericalconfiguration with an inner hemisphere 84 and an outer hemisphere 86.This analyzer provides large input solid angle and area necessary forx-ray photoelectron imaging.

An entrance aperture at 88 is receptive of the electron beams from thetransfer lens. A slotted exit aperture at 90 lies diametrically oppositethe entrance aperture.

A negative deflecting voltage relative to inner hemisphere 84 is appliedto outer hemisphere 86 from a voltage source 96 and lines 97. It will berecognized that beam trajectories between hemispheres 84,86 depend onelectron energy and applied voltage, and only beams of a selected narrowenergy range transit through exit aperture 90. Aberrations are at aminimum at the 180° hemispherical path exit.

Selecting the energy for the monochromatic electron beams is by known ordesired means, for example by adjusting the applied voltage from source96. Alternatively a conventional electron lens 98 may be used whichselectively modifies the energy of the electrons received by energyfilter 82. The lens may be integral with or constitute transfer lens 62.The energy modifying lens 98 is indicated schematically between transferlens 62 and filter 82 in FIG. 1. Electronic controlling means 100 forlens 98 in cooperation with filter 82, effect the selected energy of themonochromatic beams emerging from aperture 90. Optionally, andpreferably, the energy modifying lens is situated at 98' betweentransfer lens 62 and objective lens 20, with a controller 100'. Also oneor more lenses at 98' may be used for further magnification. Suitablelenses 98 (or 98') are disclosed in aforementioned U.S. Pat. Nos.3,766,381 and 3,617,741.

A second transfer lens 102 is positioned beyond exit aperture 90 so asto be receptive of the monochromatic beams to refocus the same. Thesecond transfer lens may be substantially identical to the firsttransfer lens but oppositely oriented. Thus the second lens 102 has anoutlet side from which the refocused beams emerge, correspondingstructurally to the inlet side of first transfer lens 62. The outletside of lens 62 would be represented by first component 64 shown in FIG.3.

The beams are then focused into a second image plane 104 with an orifice105 therein located at the focal distance of the second transfer lens. Aprojection lens 106 receptive of the refocused beams images the same ina projection plane 108 where a channel plate electron multiplier 110 islocated. Typically signals from the channel plate multiplier areconventionally detected and processed with a system (not shown)including a position sensitive device and processed for presentation asan image on a monitor or camera. Such an image shows compositionalvariations in a two dimensional area on sample surface 12, for aselected compositional element having a characteristic electron emissionenergy chosen for the monochromatic beam. The magnetic lens, andparticularly the snorkel lens, as in FIG. 2, when utilized in theorientation mode described herein and combined with the other componentsof the electron microscope according to the present invention, providesfor a wide angle pickup and focusing of emitted electrons with lowaberrations, with sufficient electron intensity for direct imaging andthus a high resolution of variation in composition along the surface.For example with X-ray photoelectrons, in an ESCA system, havingenergies of about 500 to 1000 electron volts, a surface spot resolutionof 0.5 microns is practical. For Auger electrons, which have similarenergies, the resolution should be 100-200 angstroms, being limited onlyby signal strength. Another substantial advantage of the high efficiencyis ability to use lesser local intensity of the source beam on thesurface and attendant reduced surface damage to a small spot.

The microscope is described hereinabove for mapping a plurality of spotson the sample surface. The plurality of spots generally will approach acontinuum. However, it may be desirable to collect all electrons fromthe entire area of the multiplier 108 in order to analyze the entiresurface area with a high intensity beam resulting in a high totalsignal. Alternatively the energy source 16, such as an electron beam,may be focused into a spot on the surface and, for example, may bescanned over the surface in a scanning mode. In either event, themicroscope of the present invention is advantageously utilized as ananalysis device without direct imaging, with high collection efficiencyand, therefore, sensitivity.

While the invention has been described above in detail with reference tospecific embodiments, various changes and modifications which fallwithin the spirit of the invention and scope of the appended claims willbecome apparent to those skilled in this art. The invention is thereforeonly intended to be limited by the appended claims or their equivalents.

What is claimed is:
 1. A direct imaging, monochromatic electronmicroscope including emitting means for emitting electrons from aplurality of spots across a sample surface, objective means forcollecting a substantial portion of the emitted electrons from thesample surface, collimating means for collimating the substantialportion of the emitted electrons into a plurality of groups of electronbeams, an energy filter receptive of the groups of beams to transmitmonochromatic beams having a selected energy, imaging means receptive ofthe monochromatic beams for focusing the same to effect an image of thespots, and detector means for detecting the image, the objective meanscomprising an objective lens formed of a magnetic field generatingtoroidal coil having a central hole therein with a dish-shapedmagnetically permeable member cupped coaxially over the toroidal coil,the permeable member having a neck portion extending substantiallythrough the central hole, and the objective lens being situated tocollect the substantial portion of the emitted electrons from a samplesurface interposed proximate the neck portion between the objective lensand the collimating means.
 2. An electron microscope according to claim1 wherein the energy filter comprises a spherical analyzer ofhemispherical configuration with an entrance aperture receptive of theelectron beams from the collimating means and a slotted exit aperturelocated diametrically opposite the entrance aperture.
 3. An electronmicroscope according to claim 1 further comprising means for selectingthe energy for the monochromatic electron beams.
 4. An electronmicroscope according to claim 1 wherein the emitting means comprises anelectron gun directed at the sample surface to cause Auger electronemission from the sample surface.
 5. An electron microscope according toclaim 1 wherein the emitting means comprises an X-ray source directed atthe sample surface to cause photoelectron emission from the samplesurface.